The invention relates to two-dimensional spectrometers, wherein light to be analyzed enters an aperture and is subjected to cross-dispersion involving use of an echelle grating, and wherein a reflecting optical system images the aperture as a dispersed-light pattern at a focal plane.
Pending Tobias application Ser. No. 450,027, filed Dec. 13, 1989, contains an introductory discussion of spectrometers, and reference is made to said application for a background statement, particularly with respect to attempts to use an echelle grating and a two-dimensional detector array at the focal plane of the system. Bilhorn, et al. papers in Applied Spectroscopy (Vol. 41, No. 7, 1987, at pages 1125 to 1135; and Vol. 43, No. 1, 1989 at pages 1 to 11) are disclosed as representative of the state of the art, namely, a spectrometer as a single-pass system, from light source to image plane, with an echelle grating and a prism to produce cross-dispersion, and an off-axis Schmidt camera to reduce the size of the focal-plane image to match the dimensions of a two-dimensional detector array. The primary disadvantages of such a spectrometer are its slow optical speed, and its large, clumsy and expensive construction. The Tobias application seeks to avoid such disadvantages by providing a single catadioptric system as the collimator and the camera for two-pass use of a variety of two-dimensionally dispersed spectrometers, and off-axis aberration effects are minimized by arranging a fiber optic as the light-entrance aperture, on the central optical axis of the spectrometer and in close proximity to, or centrally inserted in a two-dimensional array detector at the image plane.
But neither the single-pass system of Bilhorn, et al. nor the two-pass system of Tobias addresses the problem of chromatic aberration, which is inherent in both systems.